Field of the Invention
The present invention relates to a camshaft as well as a manufacturing method therefor, in which the relative positioning between outer cams and inner cams can be varied in order to arbitrarily control the opening angles and opening times of engine valves.
Description of the Related Art
As a camshaft for opening and closing engine valves provided in cylinders of an internal combustion engine, there are known, for example, as proposed in International Publication Nos. WO 2011/089809 and WO 2012/090300, devices by which the relative positioning between outer cams and inner cams are made variable in order to arbitrarily control the opening angles of the engine valves.
More specifically, a camshaft is equipped with a cylindrical outer shaft on which outer cams are provided on the outer circumference thereof, and an inner shaft, which is arranged rotatably in the interior of the outer shaft. Notches having shapes along the circumferential direction thereof are formed on the outer shaft, whereas, through the notches, inner cams are fixed to the inner shaft from the outer side. Therefore, when the inner shaft is rotated relatively with respect to the outer shaft, the inner cams rotate in following relation with the inner shaft (in so-called co-rotation therewith), and slide in the circumferential direction along the outer circumferential surface of the outer shaft. Consequently, the relative positioning between the outer cams and the inner cams can be made variable.
With the camshaft, fixing of the inner cams with respect to the inner shaft is carried out using pins. More specifically, pin holes are provided in the inner shaft that extend along diametrical directions thereof, and insertion holes are formed in the inner cams. In addition, the inner cams are fixed with respect to the inner shaft by press-fitting the pins, from a diametrical direction of the inner cams, into the pin holes through the insertion holes and the notches.
At this time, there is a concern that if the inner shaft were to become bent inside the outer shaft due to frictional resistance upon press-fitting the pins into the pin holes, the outer circumferential surface of the inner shaft would become fixed in a state of being pressed into contact with the inner circumferential surface of the outer shaft. In such a case, frictional resistance is generated mutually between the outer shaft and the inner shaft accompanying relative rotation between the outer shaft and the inner shaft. For this reason, there is a concern that rotation of the respective members would be hindered, the accuracy in adjusting the relative positions of the outer cams and the inner cams would be decreased, and the contact surfaces of the outer cams and the inner cams may become worn, whereby the durability of the camshaft is deteriorated.
Thus, with the camshaft disclosed in International Publication No. WO 2011/089809, the inner diameters of the pin holes are made greater than the diameters of the pins, and frictional resistance that occurs when the pins are inserted into the pin holes is decreased. In this case, both end portions in the axial direction of the pins that penetrate through the pin holes are caulked, and large diameter portions (stopper portions) are formed thereon, whereby the pins are fixed with respect to the pin holes.
Further, with the camshaft disclosed in International Publication No. WO 2012/090300, in order to prevent fixing thereof in a state in which the outer circumferential surface of the inner shaft is pressed into contact with the inner circumferential surface of the outer shaft, after the pins have been press-fitted into the pin holes, the pins are moved in directions opposite to the directions in which the pins were press-fitted. More specifically, pins are used which are composed of a small-diameter portion and a large-diameter portion, and a rod-shaped pushback tool is used together therewith. The small diameter portions of the pins have diameters of a size adapted to be press-fitted into the pin holes, whereas the large diameter portions have larger diameters than the inside diameters of the pin holes. Further, a stepped portion is formed mutually between the small diameter portion and the large diameter portion.
More specifically, initially, the small diameter portion of the pin is press-fitted from one end side of a pin hole that penetrates through the inner shaft, and the stepped portion is brought temporarily into abutment against the outer circumferential surface of the inner shaft. Next, the pushback tool is inserted from through holes that are formed in the inner cam and the outer shaft, so as to face the other end side of the pin hole, whereupon an end surface of the small diameter portion is pressed thereby. In accordance therewith, together with moving the pin in a direction opposite to the direction in which it was inserted, the relative positioning of the inner shaft with respect to the outer shaft is adjusted, and a clearance is formed mutually between the inner shaft and the outer shaft.